The present invention generally relates to electric induction melting furnaces, and more particularly to a multi-chamber furnace wherein a molten composition, or melt, is heated and optionally melted in a first chamber, and a metered quantity of the melt is discharged from a second chamber.
Handling and melting a material that burns in the presence of oxygen, such as a magnesium based composition, presents special process control problems. For example, at around 435xc2x0 C. (nominal incipient melt temperature) and above, molten magnesium reacts violently with air by combustion supported by oxygen in the air. At the same time, these types of alloys are finding increased use. For example, in the automotive industry, lightweight magnesium alloy components, die cast or otherwise produced, provide a lighter vehicle with a higher fuel economy.
Early induction melting of magnesium alloys was typically accomplished in an induction furnace of the type illustrated in FIG. 1. Furnace 100 comprises a crucible 102, thermal insulation 103, induction coils 104a, 104b and 104c, magnetic shunt assembly 108, and tilting mechanism 110. Crucible 102 was formed from a material that would not chemically react with the molten magnesium alloy 112 in the crucible. An open space 114 was provided between crucible 102 and thermal insulation 103 to allow for the drainage of any molten material that might leak from the crucible. The leakage could be removed from the furnace by removing plug 116 and draining the material. Coils 104a, 104b and 104c were individually controlled, and were powered from a utility source operating at 50 or 60 Hertz. The general configuration of the interior of the cylindrical crucible was a relatively large height and a small diameter since magnetic coupling of the field generated by current flowing in the coils was mainly with the crucible 102, although some magnetic flux penetrated into the molten magnesium alloy (melt) to provide a relatively small amount of direct induction heating and magnetic stirring of the melt. However, most heating of the melt was accomplished by conduction from the inductively heated crucible 102. Coils 104a, 104b and 104c were selectively energized on the basis of the height of the melt in the crucible at any given time. Magnesium alloy billets were used as feedstock for the furnace and lowered into the melt by a suitable transport system. The furnace operated as a hot heel furnace in which a minimum amount (heel) of molten magnesium alloy was always left in the crucible to facilitate the conduction heating of a billet that was added to the crucible. As mentioned above, molten magnesium reacts violently with oxygen in the air. Consequently, either a cover flux or protective atmosphere was placed over the exposed surface of the melt. Cover fluxes are low melting mixtures of salts that melt and flow over the surface of the melt to form a film that reduces vaporization and oxidation. However, fluxes create a corrosive atmosphere and can cause corrosion problems in castings that are poured from the molten magnesium alloy. Protective atmospheres are generally mixtures of air with sulfur dioxide, or carbon dioxide and/or sulfur hexafluoride, and are commonly used to modify the oxide film formed on the surface of the melt to suppress vaporization and further oxidation. As an alternative to using a protective atmosphere to form a surface oxide coating, an inert gas, such as argon or helium (provided that the protective volume is enclosed for this lighter than air gas), can be used to prevent magnesium from burning by excluding air from the surface of the melt. Tilting mechanism 110 was used to pour the melt from the crucible for casting. The pour, and also the addition of feedstock billets, must be very carefully performed to minimize disturbance of the protective flux or atmosphere that is provided over the surface of the melt in the crucible. In an alternative method for tapping the melt, a siphon tube is immersed in the melt to draw a volume of molten magnesium alloy for a casting pour. However, the siphon tube process requires penetration of the melt""s surface. Further, the weight of the tube and the melt contained in the siphon presents a significant handling task in movement of the tube from within the melt to a receptacle in which the melt is released.
U.S. Pat. No. 5,908,488 (the 488 patent), entitled Magnesium Melting Furnace and Method for Melting Magnesium, illustrates another approach to melting and pouring magnesium for a casting operation. The furnace (1) in the 488 patent, which is configured to operate as a combustion furnace, comprises a horizontally oriented multi-chambered furnace consisting of a melting chamber (2), a holding chamber (4) and a meter chamber (6). Magnesium feedstock is added to the melting chamber in which it melts and flows to the holding chamber. In the holding chamber, impurities filter out of the melt and the magnesium melt flows to the meter chamber. A protective atmosphere of an air/sulfur hexafluoride mixture is used over the surfaces of the melt in the chambers. A mechanical metering pump (27) lifts molten metal out of the meter chamber and into a transfer pipe (28) that transfers the melt to a die casting machine or a transport container. The mechanical metering pump represents an improvement over pouring or siphoning the molten magnesium from the furnace but introduces a mechanical component that is subjected to a harsh operating environment and is largely recognized as practically ineffective, expensive, unreliable and, consequently, in need of frequent maintenance.
It is an object of the present invention to provide an induction furnace that will safely melt and heat molten metals, including molten metals that react violently with air, and provide a metered draw of the melt from the furnace in a clean and efficient manner.
In one aspect, the present invention is an apparatus for, and method of, heating a melt in a furnace and providing a metered discharge of the melt from the furnace. The furnace comprises a melt chamber and a meter chamber that are interconnected by a passage.
In one example of the invention, a melt chamber stopper means can either allow or inhibit flow of the melt between the melt and meter chambers through the connecting passage. In another example of the invention, a valve is used to either allow or inhibit flow of the melt between the melt and meter chambers through the connecting passage. In one example of the invention, a meter chamber stopper means can either allow or inhibit flow of a metered discharge of the melt from the furnace. In another example of the invention, a valve is used to either allow or inhibit flow of a metered discharge of the melt from the furnace.
In one example of the present invention, a meter chamber stopper rod that is connected to the meter chamber stopper means is disposed within a melt chamber stopper rod that is connected to the melt chamber stopper means, and the space between the meter chamber stopper rod and melt chamber stopper rod provides a flow path for a gas that is injected into the melt in the furnace. When the furnace is in the heating state, flow of melt between the melt and meter chambers is allowed, and flow of a metered discharge of the melt from the furnace is inhibited. In this state, the injected gas bubbles through the melt in the melt chamber to the space above the surface of the melt in the melt chamber where it collects to form a protective gas blanket over the melt from oxygen in the air. When, the furnace is in the metered discharge state, flow of melt between the melt and meter chambers is inhibited, and flow of a metered discharge of the melt from the furnace is allowed. In this state, the injected gas flows into the meter chamber to flush the metered volume of melt from the chamber.
In the example of the present invention wherein valves are used to control the flow of the melt, gas is injected into the melt chamber and meter chamber by a controlled gas supply system. Means for supplying feedstock to the melt chamber are also provided. Other aspects of the invention are set forth in this specification and the appended claims.